Deprecated tox -downloadcache option removed
Deprecated tox -downloadcache option removed

Caching is enabled by default from pip version 6.0

More info:
https://testrun.org/tox/latest/config.html#confval-downloadcache=path
https://pip.pypa.io/en/stable/reference/pip_install/#caching

Change-Id: Ia80e4e66a30a3f120e19fa7f06339b1fc0c2b1c8

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar